Indigo is not dissolved in water. When the indigo is reduced under alkaline conditions, the indigo is changed into a leuco compound which is soluble to have affinity for various fibers. The water-soluble structure is oxidized and changed into an insoluble structure, such that color is exhibited. In Korea, fermentation dyeing using fresh juice salt and niram has been mainly performed, and the fermentation dyeing requires a reduction (fermentation) process at the time of requiring the niram, and a method of adding carbohydrate components such as corn syrup, etc., based on lye in connection with the reduction is known. However, the reduction fermentation conditions are strict and complicated, such that a precise technology depending on a lot of time, labor, and experience, is required.
In addition, a method for reducing insoluble indigo to water-soluble leuco-indigo may be mainly divided into a chemical reduction method using sodium hydrosulfite (Na2S2O4) which is a commercially and commonly available and strong reducing agent and a biological reduction method using microorganisms, such as a traditional manner of dyeing method (Bozic et al. 2009, Enzyme and Microbial Technology 45(4):317-323). However, the biological reduction method using microorganisms is a time-consuming process and is difficult to apply in actual industries since specific microbial species or amounts are not defined. Therefore, the chemical reduction process using the strong reducing agent such as sodium hydrosulfite or sodium sulfide (Na2S) is used in actual industrial applications (Bozic et al. 2010, Applied Microbiology and Biotechnology 85(3):563-571).
However, the chemical reduction using the reducing agent such as sodium hypochlorite or sodium sulfide causes a number of environmental problems and coloring instability, and has strong reducing power, such that when the reducing agent is discharged to an external environment, it may seriously harm an aerobic environment, and thus, various problems may occur in waste water treatment (Aino et al., 2010, FEMS Microbiology Ecology 174(1):174-183).
Above all, the reduction process is difficult to be standardized, which is the biggest obstacle to modernizing traditional indigo dyeing.